The direct-gap III-V nitride semiconductor family and its alloys span the widest spectral range of band gaps (Eg) among all semiconductors, ranging from the infrared (InN, Eg=0.7 eV) through the visible and the ultraviolet (UV) (GaN, Eg=3.4 eV) to the deep UV range (AlN, Eg=6.2 eV). This property is the basis for its applications in short-wavelength lasers and in light-emitting diodes (LEDs) for solid-state lighting applications. In addition, the wide band gaps, availability of heterojunctions, high electron-saturation velocities, and high breakdown fields enable high-speed and high-power electronic devices. Compact short-wavelength, solid-state light sources will enable a wide range of applications such as high-density optical data storage, water treatment, sterilization of medical equipment, UV-enabled security marks on credit cards and currency bills, and biological and cellular imaging.
Currently, the III-V nitride semiconductors offer the most viable approach toward the realization of high-efficiency, deep-UV optical emitters based on semiconductors. A problem that has persisted since the early 1990s and is becoming increasingly troublesome is the high resistivity of p-type GaN and AlGaN layers. The activation energy EA of the most commonly used acceptor dopant (Mg) in GaN is ˜200 meV, several times the thermal energy kBT at room temperature (where kB is the Boltzmann constant, and T is temperature). The activation energy of acceptors increases with the band gap, reaching ED˜630 meV in AlN. For comparison, the donor (Si) activation energies are ED˜15 meV for GaN and ED˜282 meV for AlN. Thus, the thermal activation of holes is highly inefficient at room temperature for GaN and becomes increasingly problematic for higher-band-gap AlGaN and MN layers. As a result, injection of holes is a severe impediment for light-emitting devices in the UV and deep-UV spectral windows. High p-type resistance leads to excessive Joule heating of p-doped AlGaN layers for Al composition xAl≧20%. Instead, p-GaN layers must be used and absorption losses incurred in the narrower-band-gap region. Furthermore, hole reflection and trapping at heterojunction valence-band offsets block hole injection into optically active AlGaN regions and reduce the efficiency of such devices. An alternative strategy for efficient p-type doping and hole injection in wide-bandgap semiconductors is therefore highly desirable at this time.
The large ionic component of the Ga(Al)—N bonds, combined with the deviation of their equilibrium lattice structure from ideal wurtzite crystals, give rise to giant spontaneous polarization fields in III-V nitride semiconductors. In addition, the strain-induced piezoelectric component of the fixed charge in the nitrides is the highest among all III-V semiconductors. At abrupt Al(Ga)N/GaN heterojunctions, the sharp discontinuity in the polarization field leads to the formation of a bound sheet charge σπ at the heterointerface, captured by the Gauss law boundary condition σπ=(P1−P2)·{circumflex over (n)}, where {circumflex over (n)} is the unit vector normal to the heterointerface, and (P1−P2) are the polarization fields across the heterojunction. When wurtzite nitride crystals are grown along the [0001] orientation (metal or Ga-face), a positive bound polarization charge creates a high electric field and energy-band bending, such that a mobile two-dimensional electron gas (2DEG) forms at AlGaN/GaN heterojunctions without the need for intentionally introduced impurity dopants. The bound sheet-charge density can be as high as σπ˜6×1013 cm−2 at pseudomorphic AlN/GaN heterojunctions, facilitating mobile 2DEGs with a very high charge carrier density. For example, in AlN/GaN semiconductor heterostructures, the mobile 2DEG concentrations are 4×1013 cm−2. Such polarization-induced 2DEGs form the basis of nitride high-electron mobility transistors that have surpassed transistors made from any other semiconductor family in RF power performance.
The polarization fields have also been exploited to create parallel sheets of 2D hole gases in Mg-doped AlGaN/GaN multiple-quantum-well structures. Although such parallel 2D hole sheets have high conductivity in the plane of the heterojunctions, they suffer from low conductivity perpendicular to the interfaces because of potential barriers in the valence band that require transport to occur through tunneling or thermionic emission processes. Even in short-period superlattice structures, the large effective mass of holes in minibands results in low mobility and high resistance. Therefore a need exist for an alternate strategy for hole doping without potential barriers that will facilitate higher conductivities.
The same reference numerals refer to the same parts throughout the various figures.